A fixedly agreed upon grid of frequencies is used in radio technology for making the establishment of the connections easier and for making optimum use of the available transmission bandwidth. The transmission channels created by this have channel bandwidths which are matched to the type of modulation and the transmission bandwidth, and which in most cases still have a safety reserve for preventing interference by the neighboring channel because of imperfect filters, for example. The channels can be permanently used by only one transmitter and a plurality of changing receivers, or they can be flexibly assigned to different transmitters.
The known access methods for the multiple use of a transmission path, such as frequency and time multiplex methods, can also be found in optical communications technology.
Frequency multiplexing methods in particular can be implemented in the most diverse manner. There is the option, for example, to combine several optical frequency ranges or wavelengths on one transmission path in that the emissions of different transmitter on different wavelengths are separated by optical filters prior to their detection in different receiver channels.
Furthermore, the signal light to be detected can be conducted, together with the emission of an unmodulated, quasi monochromatic beam source, to a photo detector. Since optical input of a receiver is converted into electrical current in a photo detector and the electrical output generated by this is therefore proportional to the square of the optical input, an alternating current at the difference frequency of both optical waves is created when two optical frequencies are detected. It is therefore possible to employ the high selectivity of electrical filters, instead of expensive optical filters, for separating a desired signal.
Finally, the option is also provided of superimposing two optical waves of the same frequency in a photo detector, wherein the value of the photo current is determined as a function of the phase relation between the lightwaves.
Regarding detection sensitivity, this method represents one of the most efficient ones. However, the need for exactly agreed upon optical frequencies arises here, too, in order to make taking up and conducting the connection easier. Proposed solutions of this problem exist for so-called dense wavelength multiplex systems, which make use of sharp absorption points of defined molecules, which appear at lesser density in the optical frequency range (M. Guy et al., "Simultaneous Absolute Frequency Control of Laser Transmitters in Both 1.3 and 1.55 mum Bands for Multiwavelength Communications Systems", IEEE JLT, vol. 14, No. 6, June 1996, pp. 1136 to 1143).
It is furthermore described how a grid (or, plan) of optical reference frequencies can be created by the periodic transmission function of a Michelson interferometer. These can be used for setting the optical frequency of the laser employed in the transmission system. Depending on the laser type, such as a laser diode or laser-diode-pumped solid state lasers, the adjustment of a grid frequency can be performed by variation of the injection current or the temperature of the laser.
The complexity and the weight of the apparatus required for this process has a disadvantageous effect on the provision of such a frequency grid, in particular in connection with applications in space.